1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer that uses an electrophotographic scheme, an electrostatic recording scheme, or the like, and a control method for the image forming apparatus.
2. Description of the Related Art
An image forming apparatus corrects, based on a result of measuring a measurement image, the density and gradation characteristics of an image formed by the image forming apparatus, to adjust the image quality to desired quality. This process is called calibration. The U.S. Pat. No. 6,034,788 proposes calibration in which a charging voltage or a developing voltage is controlled to correct maximum image density, and a gradation correction condition is changed to correct gradation characteristics.
In recent years, the market has demanded expansion of the color gamut of image forming apparatuses. The color gamut can be expanded by increasing the single-color maximum density of each of cyan, magenta, yellow, and black. For example, to increase the maximum density of each of cyan, magenta, yellow, and black, a measurement image of each of cyan, magenta, yellow, and black is formed, and a process condition for forming an image of the maximum density is determined based on a result of measuring the measurement image using a sensor. There is, however, the problem of the measurement result of the sensor being saturated in the case where the amount of toner (the amount of applied toner) attached to the measurement image exceeds a predetermined amount. In the case where the measurement result obtained by reading the measurement image is saturated, the measurement result no longer increases monotonically even when the amount of applied toner on a sheet is increased. In calibration, the relation between a measurement result obtained by reading a pattern image and a process condition (laser power, charging potential, developing potential, etc.) used when forming the pattern image needs to be determined accurately. A failure to accurately determine the relation between the amount of applied toner and the measurement result leads to lower calibration accuracy. A possible cause of the saturation of the measurement result is that, when the amount of pigment included in the toner is greater than or equal to a predetermined amount, light cannot pass through or reflect off the measurement image.